For LTE (Long Term Evolution) standardized by 3GPP (Third Generation Partnership Project), a CoMP (Coordinated Multi-Point transmission/reception) technology has been considered. The CoMP technology is a technology in which a base station performs beamforming with a null directed to a communication terminal in communication in another cell and thereby suppresses interference.
In the CoMP technology, the communication terminal, based on a CSI (Channel State Information) reference signal received from a neighbor base station other than the base station in communication (hereinafter, referred to as a “desired base station”), determines a base station with the strongest interference power (hereinafter, referred to as a “strong interference base station”) and notifies the desired base station of the strong interference base station.
FIG. 6 illustrates an example of the mapping of the CSI reference signals in eight layers in the LTE (see NPL 1 set forth below). In FIG. 6, a horizontal axis represents time, and a vertical axis represents a frequency. Each scale of the horizontal axis represents one symbol, and each scale of the vertical axis represents one subcarrier. In the example of the mapping illustrated in FIG. 6, for two antenna ports (e.g., antenna ports 15 and 16), the CSI reference signal is arranged to the same resource elements. Therefore, the CSI reference signals are orthogonal to each other at the two antenna ports. The CSI reference signal has a length of two symbols in a time direction.
In the CoMP technology, cooperative beamforming is performed, in order to reduce the interference with the communication terminal in communication in another cell, by directing the null to the communication terminal in communication in another cell.